Such catalytic devices are also called SCR (selective catalytic reduction) catalytic devices. The deactivation of such catalytic devices has several different causes, mainly:
Clogging of the honeycomb structure, or respectively the free spaces in the catalytic device. Because of this, the stack gas does not reach the catalytic device and the clogged conduit of the catalytic device is not used for the catalytic reaction. In order to use the installed catalytic material as efficiently as possible, attempts are made to decrease the clogging of honeycomb channels or plate channels by cleaning measures, such as steam blowers in the DENOX installation or manual cleaning actions. In spite of this, some of these honeycombs, or respectively free spaces in the catalytic device, become clogged over time. With some installations the catalyst modules are removed and placed on an appropriate shaking device. The clogs are loosened by the shaking movements. In this way the stack gas again gains access to the catalytic material. The increase in activity does not constitute a regeneration, it only provides access to the clogiged catalytic material. The surface layer being formed during operation remains untouched by this cleaning step. PA1 Worsening of the gas diffusion at the surface of the wall of the catalytic device because of the growth of a thin surface layer of approximately 1 to 100 .mu.m and clogging of pores. Because of this, the stack gas can only reach the pores of the catalytic material poorly or not at all. The formation of a thin surface layer worsens the chemical transformation of NO.sub.x and NH.sub.3 into N.sub.2 and H.sub.2 O, because the gas diffusion into the catalytic material is greatly hampered. PA1 Clogging of the active catalytic centers on the surface of the catalytic devices by means of the accumulation of so-called catalytic poisons, for example As, K, Na. The settling of catalytic poisons, such as arsenic, for example, on the active centers of the catalytic device makes the reaction at these centers impossible and in this way also aids in a reduction of the activities of the catalytic material. PA1 Abrasion of catalytic material by solids, such as fly ash, contained in the stack gas. The catalytic material is reduced because of the loss of catalytic material and therefore of the surface available for the reaction. The abrasion of catalytic material is an irreversible process which results in a permanent loss of activity. The following actions can also simultaneously occur in the course of abrasion by fly ash:
Removal of catalytic material and of an existing surface layer, PA2 Retention of components of the fly ash and therefore formation of a fresh gas diffusion-hindering surface layer.
A method is described in German Patent, DE 38 16 600 C2, in which the regeneration of catalytic devices contaminated by arsenic is described. This method does not take into consideration the portion of the deactivation by a gas diffusion-hindering surface layer. Aqueous solutions of nitric acid, hydrochloric acid, sulfuric acid or acetic acid are employed as the scrubbing suspension in the method according to the noted German Patent. These scrubbing suspensions have the disadvantage that for one they are too expensive and also that the disposal of the acids contaminated by arsenic is elaborate.
A method is described in European Patent, EP 0 136 966 B1, in which initially the dust adhering to the surface is removed with dry steam. The catalytic poisons are then intended to be dissolved and rinsed out in a second step by wet steam with a moisture content of .ltoreq.=0.4. Drying is performed with dry steam again. In the method in accordance with this European Patent, the thin, gas diffusion-hindering layer is not removed in a first step, instead clogged conduits are merely opened again. This has already been done on a large-scale basis for a long time in the form of socalled dust or soot blowers. The second step of this method can have an activity-increasing effect only with catalytic devices wherein the gas diffusion-hindering layer does not exist over the entire surface or not at all. Also, the generation of large amounts of dry and wet steam is very energy-intensive.
A regeneration method for deactivated catalytic devices is described in Japanese Patent, JP-A-63 147 155, wherein the catalytic devices are removed, placed into a basket, which is suspended in a regeneration treatment tank. A flow of regenerating fluid exts in the tank. The fluid consists of a suspension of an abrasive powder in water or in an acid scrubbing fluid.
Japanese Patent, JP-A-52 027 091, describes a regenerating method, in which catalytic devices with reduced performance are treated with water with a diluted aqueous inorganic acid.
U.S. Pat. No. 4,210,628 describes catalytic devices for nitrogen removal made of pulverulent or granular activated charcoal with catalytic metal additives (W, MO, V, CU, etc.). When the catalytic activity is reduced because of the formation of ammonium sulfate, the pulverulent catalytic products, which have been packed into a column, are regenerated in that either N2 is passed through them at 350.degree. C. or they are washed with hot water at approximately 80.degree. C.
A method for the reactivation of catalytic devices is described in German Patent, DE 30 20 698 C2, which removes the deactivating substances by means of a defined pressure and a defined temperature. Various gases, for example methane, propane, carbon dioxide or argon can be added in the process for optimizing thus method. This method also does not consider the gas diffusion-hindering surface layer.
A great disadvantage of most of the mentioned methods is the fact that they can only be performed in a separate installation. To this end the removal of the catalytic devices and therefore an outage of the installation is required.